A weblog for professionals in electrical, electronic, mechanical and software engineering with content provided by the members of the Long Island Consultant's Network.

December 2015

December 29, 2015

In my last posts (some time ago) I was happy that I had found a formula for the rate of water vapor transmission through a layer of still air. I have since learned that this information was hiding in plain sight in my 1985 ASHRAE Handbook of Fundamentals. It is called permeability. It is 120 grains per hour per square foot for a vapor partial pressure gradient of one inch of mercury per inch of layer thickness. There are 7000 grains per pound. After converting all these units to metric, I got close to the vapor permeability given in that internet download.

Now I have a plausible approximation for the rate of evaporation from solar-heated evaporator rafts placed in Washington rivers like the Methow and Twisp, where devastating wildfires have raged. That's nice, but now there's the question of what happens to the water vapor. How will it spread out? Where will it go? Will it make a useful change in the wildfire threat? Will the prevailing winds keep it close to the ground or will it rise into the wild blue yonder? Answer one question and a whole bunch more show up.

December 26, 2015

To maybe better appreciate the significance of human presence in the cosmos, we can take a look at some gross assumptions and a few numbers.

I've seen popular press articles on astronomy in which numbers get bandied about rather cavalierly along the lines of there being one hundred billion stars per galaxy and there being one hundred billion galaxies in the observable universe. Even if those numbers are substantially off target, they lead to an interesting consequence.

Using exponential notation, one hundred billion is one hundred times one billion which is 1E2 times 1E9 which is 1E11. Then, one hundred billion stars per galaxy times one hundred billion galaxies comes to 1E22 stars. Are you with me so far?

Now let's say that someone is born and, like George Burns, lives to be one hundred years old. Let's further say that this hypothetical someone was super precocious from birth and by some means or other, immediately started at birth contemplating individual stars at the rate of one star per second.

We're going to ignore the challenges of categorization and record keeping here and also having to do without sleep or food. Somehow, this kid manages to do it without any interruptions and without ever pausing to rest.

We come up with the following tabulation showing that this person will have contemplated somewhat more than three billion stars in the course of a century long lifetime.

That's a lot of stars maybe, but it is a microscopically small percentage of all of the stars that stood ready to be contemplated, never mind about their various properties and their associated planets, moons, comets, asteroids and whatever else.

December 24, 2015

The classic scene of a family on a long car drive to somewhere is to have the children in the back seat ask over and over again "Are we there yet??"

"Are we there yet"": is followed by "No, we still have a long way to go." which is followed by "Are we there yet??" which is followed by "No, we're only half way there." which is followed by "Are we there yet?? which is followed by "No........." and so on and so forth leading eventually up to the driver's (read that as Daddy's) utter exasperation and the sounds of childish glee coming from the car's rear seat.

Hey! That's the classic stuff of comic strips and stand-up comedy routines and there seemed to be no way of ending it until on one trip, I did find a way.

We were taking a multi-hundred mile, summer drive to upstate New York when the dreaded question started coming, "Are we there yet?" to which after several cycles of the above, I replied "Yes! We're there, but you see, (and there was this silent pause) Grandpa has this really long driveway!"

There was a loud groan from the rear seat, but the ensuing peace and quiet made the rest of the drive endurable and in subsequent drives, there was the bonus that I never heard that question again. ( Heh, heh. )

December 23, 2015

My sister sent this template to me tucked nicely into a holiday greeting card. She found it in a desk that our mother used to use. Somehow, Mom had tucked it away in there and there is where it has bided its time for fifty years. In effect, Mom's desk had been serving as an unintentional time capsule.

Look closely at this thing. It has guides for drawing a whole variety of vacuum tubes including a CRT (a cathode ray tube for all you youngsters out there) and even for constructing a schematic symbol for a transistor, just in case you happened to need one of those things.

Now look at one of the tricks you can do with this thing:

Notice there are no logic gates, no MOSFETs, no IGBTs, ....... Was life really simpler back then?

December 18, 2015

With our recent turn of the weather, we may notice that ice can do some interesting things.

Therre is a fairly new parking facility at a shopping mall near here with a driveway that leads to an upper parking level. During the past winter, that driveway was rendered useless by an extensive ice sheet. Notice the stanchions that were put in place to close off the entrance.

How this ice-over will be dealt with this winter and in winters yet to come, I cannot imagine.

Of course, by the time June came around, everything was cleared so maybe this driveway is only going to be serving us on a seasonal basis.

For something really odd though, take a look at the inverted icicles growing upward from the lower edges of a car's two doors. Cool, huh??

December 11, 2015

There is much with which to find fault in LinkedIn's new interface. Significant user capability has been removed such as no longer being able to examine what used to be called, I think, "Postings You've Started" and which were awaiting group manager approval before being allowed to appear.

What happens now instead is this:

Your being made aware of "moderation" is an ephemeral event at best.

Since many group managers appear to have abandoned their groups (Some SWAM'ed essays that I have posted on some groups have languished in SWAM limbo for well over a year.) and since your SWAM'ed essays are no longer accessible to you, you can pretty much count of having much of your work destined for indefinite abeyance.

December 10, 2015

In my Dec. 8 post, I used an idea credited to I.S.Bowen in the 1926 Physical Review. In my customary dilettante fashion, I have guessed at the contents of the paper without reading it (That's on my to-do list.) and modeled the atmosphere above the heated water surface above the evaporator raft as a still-air layer with the wind-driven and convecting air above it. I was able to derive the thickness of the still-air layer by equating the convection heat transfer rate to the heat conduction through the air layer. This turned out to be 0.56 cm with zero wind speed. The layer would be thinner with wind present.

The partial pressure of water vapor immediately above the water surface is the partial pressure of saturated (100% relative humidity) water vapor at the surface temperature. The partial pressure of water vapor at the top of the still-air layer is the saturated vapor pressure at the ambient air temperature times the relative humidity, which can be less than 10% in our western states during the summer. The difference in vapor pressure between the bottom and top of the still-air layer divided by the thickness of the layer is the vapor pressure gradient. In my previous post, I wondered if the pressure gradient might not be uniform through the air layer. But in the steady state, which concerns us here, the vapor pressure gradient must be constant throughout the layer because the vapor mass transport rate must be constant across the layer. There are no internal sources or sinks inside the layer.

With some more reading, learning, and organizing/mechanizing the calculations, I may be able to produce performance calculations that might indicate economic feasibility of solar-heated evaporator rafts for reducing the wildfire threat.

December 08, 2015

I took a GoogleEarth trip along the Methow and Twisp rivers, where there were many wildfires this past summer. At high-enough scale, the waterways were well defined, except for junctions where water was cascading over rocks. The water was flowing in valleys, where the wind speed would be low, but the sunlight would be adequate with a few trees removed. I have not learned what boat traffic to expect, but it seems that there is space for evaporator rafts. In my last blog, I did not know how to calculate the evaporation rate from a raft that is sheltered from the wind, but literature on Bowen's Ratio has shown me how to proceed.

There are commonly-used formulas for heat transfer by convection from a warm surface to ambient air. One given by Sartori multiplies the difference between surface and ambient temperatures in deg C by 5.7 to calculate the heat transfer in W/sq meter with no wind velocity. If we assume that there is a layer of still air between the warm horizontal surface and the convection-driven air that carries the heat away, the same heat transfer rate must be driven by conduction through the still air in this layer. The heat conductance of still air is 0.032 W/mC, so dividing this by 5.7 W/m squared C gives a thickness of 0.0056 meter or 0.56 cm for the still air layer.

In the case of an evaporator raft, the warm surface is water heated by the solar absorber just below the water surface. If we know the water surface temperature, we can look up the saturated vapor pressure for water at this temperature. The air just above the water will be saturated (100 % relative humidity) and have this same vapor pressure. If the temperature of the water-air interface is 40 C, the vapor pressure is 0.074 bar or 0.074 ( 10^5) Pascal. If the ambient air above the still air layer is 60F with 10% relative humidity, its vapor pressure is 0,00176 bar. The differential in vapor pressure across the still air layer is 0.072 ( 10^5) Pascal.

I got lucky and found a website that gave me a formula for the water vapor diffusion coefficient for water vapor in air. This is the ratio of water vapor transport in kg/m squared sec to the vapor pressure gradient in Pascals/meter. Assuming that the vapor pressure variation in the still air layer is linear, I calculated a water vapor yield of 0.89 kg per sq meter per hour. This seems consistent with evaporation rates I have calculated for other configurations, but I will have to do a lot more calculation for consistent sets of parameters and variables, and also figure out how the vapor pressure changes through the still air layer, probably not linearly. Also, the diffusion coefficient used above is certainly not the same as the diffusion coefficient given in the 1957 American Institute of Physics Handbook. There is also something called diffusivity. I've got some learning to do.